A series of experiments are proposed to analyze the nature and biological function of early cell polarity in a model deuterostome, the sea urchin cleaving embryo. The animal kingdom is divided into two major groups based on how their embryos develop. Echinoderms, including sea urchins, develop like lower and higher chordates whereas other genetic model organisms heavily studied for their development, including flies and worms, develop very differently. The embryos of these lower organisms exhibit very profound cellular and embryonic polarity upon fertilization and early cleavage divisions whereas such polarity has not been well investigated in the deuterostomes. While the echinoderm embryo has stereotypical cleavage, with for instance an eight cell embryo composed of two tiers of four cells, this embryo also exhibits formation of small micromeres at the vegetal pole at the 16 cell stage whose developmental roles as signaling cells for gastrulation and as cells fated to form the larval skeleton are well studied. Less well analyzed is the cellular polarity of this early embryo at the earliest cleavage divisions. Our preliminary work strongly suggests that the same polarity proteins, termed the Par complex, are polarized, as in the protostomes, at the first cell division in echinoderms and that the polarity is functional. We ask questions in this proposal about how the Par complex proteins are anchored to the cell cortex in a polarized manner and how their functioning is regulated in this model deuterostome. PUBLIC HEALTH RELEVANCE: The research proposed in this application deals with the earliest stages of an embryo's development. We are studying the formation of early cell polarity after fertilization and the first cell divisions in a model organism closely related to vertebrates and findings from this research are directly relevant to the way in which humans develop.